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United States Patent |
5,093,605
|
Meinertz
|
March 3, 1992
|
Power regulation during start up and shut down
Abstract
A switched mode power supply and horizontal deflection system comprises a
first oscillator circuit for generating horizontal rate synchronizing
trigger pulses, having a voltage supply input terminal; a horizontal
output stage; and, a second oscillator circuit for driving the output
stage, operable at a horizontal rate responsive to the trigger pulses and
free running at a different rate absent the trigger pulses. An overcurrent
protection circuit for the horizontal output stage responds to an
overcurrent condition which can occur during free running of the second
oscillator circuit. A flyback transformer is coupled to the horizontal
output stage and has a secondary side voltage supply coupled to the
voltage supply input terminal for energizing the first oscillator circuit
during operation of the output stage. An energy storage device, for
example a large value capacitor, is coupled to the voltage supply input
terminal for energizing the first oscillator circuit for a period of time
after the horizontal deflection system is deactivated. The capacitor and a
resistor form a timing network for the first oscillator circuit. The first
oscillator circuit continues generating synchronizing trigger pulses and
prevents operation of the second oscillator circuit at the free running
frequency. A quick charging path for the energy storage device, for
example a Zener diode in parallel with the resistor, minimizes operating
time of the second oscillator circuit at the free running rate prior to
the initiation of the synchronizing trigger pulses when the power supply
and horizontal deflection system is activated.
Inventors:
|
Meinertz; Friedrich (Singapore, SN)
|
Assignee:
|
Thomson Consumer Electronics, S.A. (Paris, FR)
|
Appl. No.:
|
621467 |
Filed:
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December 3, 1990 |
Foreign Application Priority Data
Current U.S. Class: |
315/411; 348/730 |
Intern'l Class: |
H01J 029/74; H04N 005/63 |
Field of Search: |
315/408,411
358/190
|
References Cited
U.S. Patent Documents
5013980 | May., 1991 | Stephens et al. | 315/411.
|
Primary Examiner: Issing; Gregory C.
Attorney, Agent or Firm: Tripoli; Joseph S., Laks; Joseph J., Fried; Harvey D.
Claims
What is claimed is:
1. A power supply and horizontal deflection system, comprising:
a horizontal output stage;
oscillating means for driving said horizontal output stage, operable at a
controllable rate responsive to trigger pulses and free running at a
different rate absent said trigger pulses;
means for generating said trigger pulses;
means operable with said horizontal output stage for energizing said means
for generating said trigger pulses;
energy storage means for energizing said means for generating said trigger
pulses for a period of time after said horizontal output stage is
deactivated; and,
means for charging said energy storage means for a period of time after
said horizontal output stage is activated.
2. The system of claim 1, wherein said energy storage means comprises a
capacitor.
3. The system of claim 1, wherein said means for charging said energy
storage means comprises a Zener diode.
4. The system of claim 1, comprising a capacitor and a resistor defining a
timing network for said means for generating said trigger pulses, said
capacitor forming said energy storage means.
5. The system of claim 4, wherein said means for charging said energy
storage means comprises a Zener diode in parallel with said resistor.
6. The system of claim 1, further comprising means for inhibiting operation
of said oscillating means and said horizontal output stage responsive to
an abnormal current condition in said horizontal output stage.
7. The system of claim 1, comprising a standby power circuit for energizing
said oscillating means independently of said operation of said horizontal
output stage.
8. The system of claim 1, wherein said means operable with said horizontal
output stage for energizing said means for generating said trigger pulses
comprises a flyback transformer coupled to said horizontal output stage
and rectifying means for developing a derived secondary side voltage
source.
9. The system of claim 8, comprising a standby power circuit for energizing
said oscillating means independently of said operation of said derived
secondary side voltage source.
10. The system of claim 9, wherein said means for generating said trigger
pulses comprises a voltage supply input terminal coupled to said derived
secondary side voltage source, said energy storage means and said means
for charging said energy storage means.
11. The system of claim 10, comprising a capacitor and a resistor together
defining a timing network for said means for generating said trigger
pulses, said capacitor forming said energy storage means.
12. The system of claim 11, wherein said means for charging said energy
storage means comprises a Zener diode in parallel with said resistor.
13. The system of claim 1, wherein said means for generating said trigger
pulses comprises second oscillating means synchronized with a horizontal
synchronizing component in a video signal.
14. A power supply and horizontal deflection system, comprising:
a horizontal output stage;
first oscillating means for driving said horizontal output stage, operable
at a controllable rate responsive to trigger pulses and free running at a
different rate absent said trigger pulses;
second oscillating means synchronized with a horizontal synchronizing
component in a video signal for generating said trigger pulses, said
second oscillating means forming a subcircuit of an integrated circuit,
but having a voltage supply terminal independent of other subcircuits of
said integrated circuit;
means operable with said horizontal output stage for energizing said second
oscillating means;
energy storage means for energizing said second oscillating means for a
period of time after said horizontal output stage is deactivated; and,
means defining a charging path for said energy storage means, operable for
a period of time after said horizontal output stage is activated.
15. The system of claim 14, wherein said integrated circuit is a television
one-chip.
16. A power supply and horizontal deflection system, comprising:
a horizontal output stage, having means for generating a derived secondary
side voltage source;
first oscillating means for generating trigger pulses at a horizontal rate,
energized by said secondary side voltage source;
second oscillating means for driving said output stage at said horizontal
rate responsive to said trigger pulses, said second oscillating means free
running at a different rate absent said trigger pulses and being energized
independently of said horizontal output stage;
means operable during part of both activation and deactivation of said
horizontal output stage for preventing said free running of said second
oscillating means.
17. The system of claim 16, wherein said means for preventing said free
running of said second oscillating means comprises:
energy storage means for energizing said second oscillating means for a
period of time after said horizontal output stage is deactivated; and,
means operable for a period of time after said horizontal output stage is
activated for charging said energy storage means.
18. The system of claim 16, wherein:
said energy storage means is charged at a first charging rate by operation
of said derived secondary voltage source; and,
said means operable for a period of time after said derived secondary side
supply is activated for charging said energy storage means supplies energy
at an accelerated rate relative to said first charging rate.
19. The system of claim 17, wherein said energy storage means comprises a
capacitor and said means for charging said energy storage means comprises
a Zener diode.
20. The system of claim 19, further comprising a resistor, said resistor
and said capacitor forming a timing network for said first oscillating
means, said Zener diode being coupled in parallel with said resistor.
21. The power supply of claim 16, further comprising means for disabling
said power supply responsive to detection of a current condition which can
occur during said free running of said second oscillating means.
22. A power supply and horizontal deflection system, comprising:
a horizontal output stage;
oscillating means for driving said output stage, operable at a controllable
rate responsive to trigger pulses and free running at a different rate
absent said trigger pulses;
means for generating said trigger pulses;
means for storing energy;
means for transferring energy from said horizontal output stage for
energizing said means for generating said trigger pulses and for charging
said means for storing energy, said energy storage means energizing said
means for generating said trigger pulses for a period of time after said
horizontal output stage is deactivated; and,
alternate means for transferring energy from said horizontal output stage
to said energy storage means for a period of time after said horizontal
output stage is activated.
23. The system of claim 22, wherein said energy storage means comprises a
capacitor.
24. The system of claims 22, wherein said alternate means for transferring
energy to said energy storing means comprises a Zener diode.
25. The system of claim 22, comprising a capacitor and a resistor defining
a timing network for said means for generating said trigger pulses, said
capacitor forming said energy storage means.
26. The system of claim 25, wherein said alternate means for transferring
energy to said energy storing means comprises a Zener diode in parallel
with said resistor.
27. The system of claim 22, further comprising means for inhibiting
operation of said oscillating means and said horizontal output stage
responsive to an abnormal current condition in said horizontal output
stage.
28. The system of claim 22, comprising a standby power circuit for
energizing said oscillating means apart from operation of said horizontal
output stage.
29. A power supply and horizontal deflection system, comprising:
a horizontal output stage;
oscillating means for supplying switching pulses to said horizontal output
stage, operable at a controllable rate responsive to trigger pulses and
free running at a different rate absent said trigger pulses;
means for generating said trigger pulses;
means operable with said horizontal output stage for energizing said means
for generating said trigger pulses;
energy storage means for energizing said means for generating said trigger
pulses for a period of time after said horizontal output stage is
deactivated; and,
means defining a charging path for said energy storage means for a period
of time after said horizontal output stage is activated.
30. The power supply of claim 29, further comprising means for disabling
said power supply responsive to detection of a current condition which can
occur during said free running of said oscillating means.
31. A power supply and horizontal deflection system, comprising:
a horizontal output stage;
oscillating means for driving said output stage, operable at a controllable
rate responsive to trigger pulses and free running at a different rate
absent said trigger pulses;
means for generating said trigger pulses;
means defining a first DC voltage source operable with said horizontal
output stage for energizing said means for generating said trigger pulses;
energy storage means defining a second DC voltage source for energizing
said means for generating said trigger pulses for a period of time after
said horizontal output stage is deactivated; and,
means defining a charging path for said energy storage means for a period
of time after said horizontal output stage is activated.
32. The system of claim 31, comprising a capacitor and a resistor defining
a timing network for said means for generating said trigger pulses, said
capacitor forming said energy storage means.
33. The system of claim 32, wherein said means defining a charging path for
said energy storage means comprises a Zener diode in parallel with said
resistor.
Description
This invention relates to the field of switched mode power supplies for
television apparatus, and in particular, to a control circuit for
preventing overcurrent operation by the horizontal output stage in a
horizontal deflection system when the television apparatus is turned on or
off.
Televisions with microprocessor control typically have certain circuits
which are continuously active in a standby mode of operation, even when
the television has been switched off. Other circuits are energized only
after the television set has been switched on, in a run mode of operation.
Problems can be encountered coordinating the interaction of systems which
are always active and those which are active only during the run mode of
operation.
The horizontal output stage in a horizontal deflection system may comprise
a horizontal output transistor driven by a sawtooth waveform oscillator. A
configuration for one such output circuit known as a Wessel circuit is
shown in accompanying drawings. A sawtooth oscillator generates the basic
driving waveform, and is typically free running at a lower frequency than
the horizontal scanning frequency. For an NTSC interlaced signal, the
horizontal scanning frequency is approximately 15,750 Hz. The free running
frequency might be between 13,000 Hz and 14,000 Hz.
A horizontal oscillator is provided for generating a synchronizing timing
signal precisely at the horizontal scanning rate, synchronized with the
video input signal. Such a horizontal oscillator may be incorporated as
one the circuits in a one-chip. Such a one-chip may be part No. M51408
available from Mitsubishi. The horizontal oscillator circuit provides
trigger pulses to the otherwise free running oscillator, to assure that
the sawtooth waveform is precisely equal to the horizontal scanning
frequency rather than the free running frequency. The sawtooth signal may
be coupled through buffer and driver stages, to the horizontal output
stage, which may be a horizontal output transistor. The horizontal output
transistor is coupled to a flyback transformer, from which a number
secondary voltage sources may be derived from energy in the flyback
pulses. Rectifying circuits may be coupled to secondary windings of the
flyback transformer for developing these voltage sources at different
voltage levels which may be required by various load circuits in the
television.
Typically, neither the sawtooth waveform oscillator nor the one-chip are
energized during the standby operation. In fact, the one-chip is typically
energized by one or more secondary voltage sources generated by the
switched mode operation of the power supply. Moreover, the switched mode
power supply, which relies upon switching of a horizontal output
transistor to develop the secondary derived voltage sources, cannot
operate until the sawtooth waveform has been generated by the sawtooth
oscillator.
It can be a characteristic of such switched mode power supplies that
sufficiently prolonged operation at the free running frequency results in
the horizontal output transistor being conductive for too long a period of
time, at each turn-on. This results in an overcurrent condition, which can
damage the horizontal output transistor and other components in the
switched mode power supply. Accordingly, a safety circuit is often
provided for sensing the overcurrent condition and disabling the power
supply. The safety circuit can be responsive to overcurrent or overvoltage
conditions having other causes as well.
Televisions with microprocessor control are programmed to undergo a certain
sequence of operations when the television is switched off, in order to
prevent undesirable or harmful transient conditions. In a switched mode
power supply for a horizontal deflection system as described above, such
an undesirable transient condition can occur when the television set is
switched off. The horizontal oscillator in the one-chip providing the
horizontal rate trigger pulses for the sawtooth oscillator can stop
functioning before the sawtooth oscillator stops functioning. This sudden
change of horizontal frequency as the sawtooth oscillator begins free
running causes a large current spike to be conducted by the horizontal
output transistor, which in turn causes operation of the safety circuit,
disrupting the orderly, soft switch off of the television.
The horizontal oscillator in the one-chip has a separate Vcc input
terminal, which is coupled to derived secondary voltage source of the
flyback transformer. The Vcc input pin of the chip requires a series
resistor for frequency control and a capacitor for filtering out ripple.
In accordance with an inventive arrangement, soft switch off can be
assured by substantially increasing the capacitance value of the filtering
capacitor, for example to 1,000 microfarads. This ensures that the
horizontal synchronizing trigger pulses will continue to be generated long
enough to maintain the horizontal frequency oscillation of the sawtooth
oscillator until the soft switch off has been completed.
Although the introduction of large capacitance filter capacitor solves the
soft switch off problem, a further problem can remain. The value of the
filter capacitor increases the R-C time constant at the Vcc input pin of
the one-chip. Whenever the television is switched on, the filter capacitor
can require so much time to charge that the sawtooth oscillator free runs
long enough at the lower frequency to cause the overcurrent condition,
which causes activation of the safety sense circuit, which interrupts
operation of the switched mode power supply. In effect, the safety circuit
can prevent the television from ever being successfully turned on. It is
necessary to significantly decrease the start up time for the horizontal
oscillator in the one-chip, without the sacrificing the filtering function
of the R-C network and without sacrificing reliable soft switch off. In
accordance with another inventive arrangement, a Zener diode can be
coupled in parallel with the resistor and bypass the resistor during
startup of the television, providing a quick charging path for the
capacitor. The Zener diode stops conducting as soon as the operating
voltage is reached, enabling the R-C network to provide ripple filtering
as before. The quick charging path reduces the time during which the
sawtooth oscillator operates at the free running frequency, and prevents
the overcurrent condition of the horizontal output transistor.
A switched mode power supply and horizontal deflection system according to
inventive arrangements taught herein assures reliable and soft turn on and
off. A power supply and deflection system in accordance with these
inventive arrangements can comprise a first oscillator circuit for
generating horizontal rate synchronizing trigger pulses, having a voltage
supply input terminal; a horizontal output stage; and, a second oscillator
circuit for driving the output stage, operable at a horizontal rate
responsive to the trigger pulses and free running at a different rate
absent the trigger pulses. An overcurrent protection circuit for the
horizontal output stage responds to an overcurrent condition which can
occur during free running of the second oscillator circuit. A flyback
transformer is coupled to the horizontal output stage and has a secondary
side voltage supply coupled to the voltage supply input terminal for
energizing the first oscillator circuit during operation of the output
stage. An energy storage device, for example a large value capacitor, is
coupled to the voltage supply input terminal for energizing the first
oscillator circuit for a period of time after the horizontal deflection
system is deactivated. The first oscillator continues generating
synchronizing trigger pulses and prevents operation of the second
oscillator at the free running frequency. A quick charging path is
established for the energy storage device, for example by a Zener diode
coupled in parallel with the resistor. The quick charging path minimizes
operating time of the second oscillator at the free running rate prior to
the initiation of the synchronizing trigger pulses when the power supply
and horizontal deflection system is activated.
FIGS. 1-4 are a composite schematic of a switched mode power supply,
employing a Wessel circuit, for a television apparatus according to an
inventive arrangement.
FIG. 5 is a schematic of the horizontal oscillator and vertical oscillator
of the one-chip for the television apparatus shown in FIGS. 1-4.
FIG. 6 is a schematic of the vertical drive circuit for the television
apparatus shown in FIGS. 1-5.
In the drawings, all capacitances are in farads and EC equals 16 volts
unless otherwise noted. All resistances are in ohms, 1/4 watt, unless
otherwise noted.
In FIG. 1, an AC mains supply is coupled to a diode bridge comprising
diodes DP26, DP27, DP28 and DP29. Half wave rectified voltage is available
as VSTANDBY, which is the source for power during the standby mode of
operation. The standby voltage is an input to a voltage regulator, for
example a series pass regulator, which supplies standby voltage to a
microprocessor, not shown. The microprocessor is responsive to on-off and
other control commands.
Transistor TP11 acts as a gate for the remaining half wave rectified pulses
from the diode bridge. Transistor TP11 is responsive to operation of
switch transistor TP12. The base of switch transistor TP12 is coupled to a
STANDBY line (FIG. 2) from the microprocessor. The STANDBY line goes high,
turning transistor TP12 on, whenever the microprocessor initiates the run
mode of operation. Half wave rectified pulses gated by TP11 provide energy
for charging capacitor CP01 up to 18 volts as determined by Zener diode
DP02. The 18 volt voltage level provides a bias voltage at the junction of
a voltage divider formed by resistors RP26 and RP05. The half wave
rectified pulses are also an input to a sawtooth waveform oscillator,
generally comprising transistors TP01, TP02 and capacitor CP03.
Transistors TP01 and TP02 are normally biased off. When capacitor CP03 is
sufficiently charged, transistor TP02 turns on. This provides base drive
for transistor TP01 which also turns on. This provides a rapid discharge
path for capacitor CP03. When capacitor CP03 is fully discharged,
transistors TP02 and TP01 turn off, enabling capacitor TP03 to recharge.
The sequence repeats cyclically. The resulting waveform is a sawtooth at
the base of transistor TP04. In a free running mode, absent trigger or
synchronizing pulses delivered to the base of transistor TP01 through
diode DP05, the sawtooth oscillator free runs at a frequency less than a
standard horizontal scanning frequency. For the component values shown,
the free running frequency is between 13,000 Hz and 14,000 Hz.
The sawtooth waveform is conducted through buffer transistor TP04 and AC
coupled to the pulse width modulating (PWM) transistor TP05 (FIG. 2)
through capacitor CP48. The signal is clamped by diode DP37 and adjusted
in amplitude by the voltage divider formed by resistors RP14 and RP15.
With further reference to FIG. 2, the conduction time of PWM transistor
TP05 is related to slope of the leading edge of the sawtooth waveform. The
on/off pulse width modulating signal at the collector of transistor TP05
is coupled through the horizontal driver circuit to the horizontal output
stage, shown at the upper left hand portion of FIG. 4. In the
configuration of a Wessel circuit, the horizontal output stage is
essentially horizontal output transistor TP10. Horizontal output
transistor TP10 drives both the power supply transformer and the flyback
transformer. Briefly, the output stage transistor in a Wessel circuit
operates with an unstabilized supply voltage, and draws from the operating
voltage source only as much power is required to maintain a constant
deflection current. The conduction time of the horizontal output
transistor is regulated to maintain constant deflection current
independently of fluctuations of operating voltage and real loads.
Transistor TP10 is coupled to the horizontal yoke BP04, the flyback
transformer LP04 and the power supply transformer LP03, as shown in FIG.
4. Raw B+ voltage originating at the diode bridge rectifier circuit in
FIG. 1 is coupled to tap 12 of transformer LP03. The raw B+ voltage is
applied across the primary winding of transformer LP03 by the switching
transistor TP10. The deflection winding of transformer LP04, retrace
capacitor CP18 and damper diode DP13 are coupled across the collector to
emitter junction of the switching transistor TP10 by a first diode DP10,
poled for conduction in the same direction as the collector to emitter
junction. A secondary winding of transformer LP03 is coupled across the
deflection winding by a second diode DP11 poled to conduct and transfer
energy from the primary winding to the deflection winding during the
retrace interval. The first half of the retrace interval is the time
during which the retrace capacitor CP18 is charged by energy in the
retrace pulse flowing from the horizontal yoke. The retrace capacitor is
fully charged at the middle of retrace, when the deflection current is
zero. Current flows from the retrace capacitor back through the horizontal
yoke during the second half of retrace, charging the linearity capacitor
CP40. Retrace ends when the voltage across the retrace capacitor CP18
reaches zero, and the damper diode conducts. The damper diode conducts
until the deflection current reaches zero. Thereafter, the damper diode
turns off. Transistor TP10 will start conducting sometime before the
deflection current reaches zero, but not after, depending upon the extent
of load losses. As the deflection current exceeds zero, the diode DP10
becomes forward biased by reason of the charge on the linearity capacitor.
This is possible because transistor TP10 will already be conducting for
the power supply function, and the cathode of diode DP10 will be only
slightly above ground. The start of conduction by transistor TP10 will not
effect the deflection current, whereby regulation of the power supply
function is independent of deflection. Conduction of the deflection
current through diode DP10 and transistor TP10 continues until transistor
TP10 is turned off, which initiates retrace.
There are two safety sense circuits associated with operation of the
transistor TP10. Emitter current is directly sensed by sampling resistors
RP30 and RP31. The voltage across the sampling resistors is an input to
the base of transistor TP08 (FIG. 1), which forms part of the safety sense
circuit described in detail below. Current in the secondary winding of
transformer LP03 is sampled by resistor RP50. The voltage across resistor
RP50 is an input to a network of diodes DP31, DP31 and DP33, resistor RP48
and capacitor CP45. If the sampled voltage is of sufficient magnitude, the
DC level of the sawtooth signal on the base of PWM transistor TP05 will be
pulled down. This will reduce the conduction time of the PWM transistor,
which will reduce the conduction time of transistor TP10. This can protect
against overcurrent conditions which might be reflected back through the
transformer LP03 and otherwise cause damage before being sensed as emitter
current of transistor TP10 by resistors RP30 and RP31.
Switched mode operation of the horizontal output transistor enables a
number of secondary voltage sources coupled to secondary windings of the
flyback transformer to be developed. One of these voltages is the B+
voltage of 104 volts, which is fed back to resistor RP08 in FIG. 1 as the
principle feedback signal for regulation of the switched mode power
supply. Another secondary supply shown in FIG. 3 is a 13 volt supply
developed by capacitor CP20 and diode DP18 coupled to pin 10 of
transformer LP04. This 13 volt secondary supply is the voltage source for
the horizontal oscillator circuit of the one-chip shown in FIG. 5. When
the 13 volt supply is running, the horizontal oscillator circuit provides
output pulses on pin 20 of the one-chip precisely at a standard horizontal
scanning rate, synchronized with the video signal input. These pulses
trigger the sawtooth oscillator circuit at the base of transistor TP01,
assuring that the sawtooth oscillator operates synchronously, at the
standard horizontal scanning frequency. Yet another secondary supply is a
22 volt supply developed by capacitor CP23 and diode DP20 for energizing
the vertical deflection driver integrated circuit shown in FIG. 6.
Operation of the sawtooth oscillator in the free running mode is
independent of operation of the horizontal oscillator in the one-chip,
energized by the 13 volt supply. In fact, the sawtooth oscillator will
operate in the free running mode before the 13 volt secondary supply
becomes available, whenever the television is switched on. Moreover, the
free running oscillator is apt to continue free running even when the
switched mode power supply has ceased operation, when the television is
switched off. The on-time of the horizontal output transistor is increased
during the free running operation of the sawtooth oscillator. The
transition from synchronized to free running is an abrupt transition
occurring as soon as the synchronizing trigger pulses stop. The sawtooth
oscillator can continue operating in the free running mode for long a
enough period of time after the horizontal oscillator in the one-chip
stops generating synchronizing pulses, for the horizontal output
transistor to operate in a overcurrent condition.
Overcurrent conditions are detected by sense resistors RP30 and RP31,
connected to the emitter of transistor TP10. When the sense voltage is of
sufficient magnitude, transistor TP08 in FIG. 1 will be turned on.
Conduction of transistor TP08 turns on transistor TP07. Together,
transistor TP07 and TP08 function in the manner of a silicon controlled
rectifier. When transistor TP08 begins conducting, its collector pulls
down the STANDBY control line through diode DP45, which turns off
transistor TP12. This in turn turns off gate transistor TP11, which
prevents further charging pulses for capacitor CP03. At the same time, a
rapid discharge path for capacitor CP01 is provided through diode DP01.
This quickly prevents further operation of the horizontal output
transistor TP10 by turning off the sawtooth oscillator and effectively
grounding the input to PWM transistor TP05. When all of the relevant
capacitors have discharged, both the base and emitter of transistor TP07
will be at a voltage level of approximately 2 diode drops below ground.
This will turn off transistor TP07, and thereafter, will turn off
transistor TP08. This will enable the STANDBY control line to go high
again and will initiate operation of the sawtooth oscillator, and
thereafter, the horizontal output transistor. Operation of this
overcurrent protection circuit is of course desirable responsive to
genuine overcurrent conditions. However, overcurrent conditions should not
be generated merely because the television set is turned on or off.
The safety sense circuit can also be activated responsive to other fault
conditions. The x-ray protection (XRP) circuit shown in FIG. 2 is
responsive to overvoltage conditions in the high voltage supply for the
cathode ray tube through diode DX03. The output of the x-ray protection
circuit is another input to the base of transistor TP08 and the collector
of transistor TP07, through diode DX01. The x-ray protection circuit
formed integrally with the one-chip is permanently disabled by grounding
pin 15. Overcurrent conditions in the vertical yoke (FIG. 6), for example
those resulting from a short circuit of S-shaping capacitor CF01, will
generate a threshold voltage across resistor RF11. This signal is also
coupled to the base of transistor TP08, through diode DF01. The vertical
yoke overcurrent signal is tapped from the AC component of the vertical
feedback (VFB) signal at potentiometer PF01. The DC component of the
vertical feedback signal is developed by the resistive divider comprising
resistors RF03, RF04 and RF05. The inventive arrangements taught herein do
not interfere with the normal operation of the safety circuit.
Accordingly, an inventive arrangement assures that the horizontal
oscillator in the one-chip will continue operating long enough to maintain
operation of the sawtooth oscillator at the horizontal scanning rate until
the soft shut down of the television has been accomplished. This is
achieved by substantially increasing the size of capacitor C121 as shown
in FIG. 5 to 1,000 microfarads. This provides a continuing energy source
for the horizontal oscillator. However, capacitor C121 requires a long
charging time. When the television is turned on, this charging time is
sufficiently long that the sawtooth oscillator operates in at the free
running frequency long enough to cause an overcurrent condition in the
horizontal output transistor, which trips the safety sense circuit. It may
be impossible to turn on the television, as the safety sense circuit keeps
turning the power supply off. This problem is solved in accordance with an
inventive arrangement by providing a quick charging path for capacitor
C121. The quick charging path is advantageously provided by Zener diode
DP47, coupled in parallel to resistor RP06. Resistor RP06 and capacitor
C121 provide filtering against ripple for the horizontal oscillator
circuit in the one-chip. The Zener diode provides a short-circuit path
around resistor RP06 when the television is turned on. This enables the
horizontal oscillator circuit in the one-chip to begin operating soon
enough to synchronize the sawtooth oscillator before the horizontal output
transistor reaches an overcurrent condition.
Other inventive arrangements may be appreciated by an analysis of the
remaining parts of the circuit schematic, which have not been described in
detail.
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